* Check return value of malloc()
* Properly handle the case where realloc() fails
* Fix NULL-pointer access reported by clang's static analyis tool

Notify that the service is beginning its shutdown (STOPPING=1) in addition
to notifying that service startup is finished (READY=1).

In the future we could perhaps add more events:
STATUS=…
ERRNO=…

Allows openfortivpn to act as system service.
The configuration is expected under $(sysconfdir)/openfortivpn/<NAME>.conf.

https://www.freedesktop.org/software/systemd/man/daemon.html#Installing%20systemd%20Service%20Files

Improve readabilty by moving conditionals that operate on 'n'
right after assignment of 'n'. The conditional at the end of
the loop:
	while (n > = 0)
has been replaced by this conditional:
	if (n < = 0)
and this one that triggers a tight 2nd loop:
	if (n == ERR_SSL_AGAIN) ⇔ if (n == 0)

The new codes *returns from the function* with ERR_HTTP_SSL upon:
	if (n < 0)
The previous code would only *leave the loop* upon:
	if (n < 0)
and then return from the function with ERR_HTTP_SSL only upon:
	if (!header)
Therefore SSL violations were silently ignored after reading
the header and while reading the body of the HTTP response.

Increase the HTTP buffer capacity when needed. You never know.
The previous size of 32 KB used to work well from 2015 to 2020.
In 2020 a case was reported where 32 KB were not enough anymore
and we increased the buffer size to 64 KB. Someday 64 KB might
not be enough either. Yet in most cases 32 KB are more than
enough. So start with 32 KB and increase well beyond 64 KB if
needed, instead of bailing out with ERR_HTTP_SSL.

These changes will help introduce 3rd party HTTP parsing code
if we decide to go that way.

* Be consistent and do not "make uninstall" $(confdir)/config
  as it is not clobbered by "make install" either.
* Silent rules for sed.

* Add missing target to "make" etc/openfortivpn/config.
* Add missing target to "make clean" etc/openfortivpn/config.
* Add missign target to "make uninstall" $(confdir)/config.

* Use particular programs through Autoconf macros.
* Create config file from template at build-time, not installation-time.
* Create a build rule for config file.
* Replace obsolete AC_GNU_SOURCE with AC_USE_SYSTEM_EXTENSIONS.

we use the same standard revision string as it is common at Snapcraft
the revision in the debug output is suppressed when there are no commits since the last tag

we do this already on mac and bsd

This special significance of '$' is why you must write '$$' to
have the effect of a single dollar sign in a file name or command.
See:
https://ftp.gnu.org/old-gnu/Manuals/make/html_chapter/make_6.html#SEC66

PR #622 has introduced new compiler warnings

* Make sure share/openfortivpn/config.template is world-readable.
* Do not comment first 2 lines.

The location of old releases has been moved from:
https://www.openssl.org/source/
to:
https://www.openssl.org/source/old/1.0.2/

CID 200508: String not null terminated (STRING_NULL)

It looks like these builds take for ever on Travis CI.

• For now we disable many warnings or even errors to avoid brutal changes to the code base.
• For now we check the whole C code base instead of checking only new patches.

when cross-compiling we should not attempt to run resolvconf,
especially not when it is set to DISABLED. In that case the whole
resolvconf code is not compiled in at all and it does not matter
how the default would be in the disabled code

* Reorganize order of operations in configure.ac in a single place.
* Consistent identation throughout the file.
* Minor changes in configure.ac documentation.

On Fedora it does not work as expected when invoked as resolvconf.
Also the exit status is not 0 so the test does not work actually:

	$ resolvconf -a -f ; echo $?
	Expected interface name as argument.
	1
	$

Check we have either HAVE_USR_SBIN_PPPD or HAVE_USR_SBIN_PPP,
not both, not none. Just in case.

While either HAVE_USR_SBIN_PPPD or HAVE_USR_SBIN_PPP *must* be defined,
HAVE_RESOLVCONF might not be defined - typically when resolvconf is not
available at build-time. RESOLVCONF_USAGE and RESOLVCONF_HELP should always
be defined, if needed as empty strings, so as to not break subsequent
printf() calls.