lldp: add option/command to override hostname

Allow to override the system name (hostname) by using '-N' as parameter to
lldpd, or by using "configure system hostname <NAME>" to lldpcli.

Signed-off-by: Jonas Johansson <jonasj76@gmail.com>
lldpd: Remove the '-N' command-line flag

Too many flags being added, use of configuration file is prefered.

Signed-off-by: Jonas Johansson <jonasj76@gmail.com>
Merge pull request #60 from jonasj76/override-hostname

lldp: add option/command to override hostname
lldpd: add a note about command-line options

lldpcli should be preferred for new options.
lldpcli: use a previously unused variable
compat: fix use of vasprintf (previously _vasprintf)
compat: include stdarg to get va_list
compat: fix vsyslog to rely on the result of vasprintf
configure: hide errors when systemd is not present
osx: blacklist p2pX interfaces

They are related to AirDrop and transmitting anything on it will broke
the wireless network. This should fix #61.
configure: make some flags conditional

Use of _XOPEN_SOURCE is quite dangerous.
NEWS: add several items to the changelog
version: prepare new release
lldpcli: document new "configure system hostname" directive
osx: new release
configure: use -fstack-protector-strong if available
configure: enable use of colored output for gcc
osx: update Homebrew formula to match what has been done in Homebrew
lldpcli: return success when reading commands from files

When reading commands from files, we chose to not stop on errors (the
main reason is to allow to specify nonexistent configuration files and
allow lldpd to still work after incompatible updates). However, we also
always return a failure status. Fix that.

Thanks to Brian Bidulock for spotting that. Closes #62.
configure: don't use fallback values in summary

They are not needed anymore, if default values are used, they are expanded.