ipsec: Only set PATH if it is not already set

The comment indicated this but it was always set anyway.  All internal
commands are called via their absolute paths, so the script only uses PATH for
the uname command, but if that is not located in one of the configured
directories the script will fail.

Also, since the internal commands are called via their absolute paths there is
no need to add the directories to PATH.
ipsec: Remove duplicate check for internal commands
ipsec: Update usage output
ipsec: Document missing commands
ipsec: Remove unsupported listcards and rereadgroups