nhrpd: lookup appropriate ipsec path

nhrpd: lookup appropriate ipsec path

lookup appropriate ipsec path. there are systems where the path where
the charon.vici file is not in standard paths. For that, 'ipsec
--piddir' may help in solving the path.

result of ipsec --piddir is as follow for example:
'
/etc/ike/ipsec.d/run
'

Note that the assumption is done that even if there are several
instances of strongswan across the vrfs, the charon.vici path file is
the same across vrfs. Consequently, as there is a thread per vrf that
performs vici initialisation, and file path retrieval is part of the
vici initialisation procedure, in order to avoid intempestive system
calls, use a boolean 'vici_charon_filepath_done' to avoid doing
unnecessary calls.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>