Wrapper to use custom p4 scripts.

Wrapper to use custom p4 scripts.

Script which allows overriding Perforce commands or adding new commands.
If the file opt/p4/p4-<command> exists then "p4 <command>" will run the
file in preference to the builtin command.

Source opt/p4/common to identify the arguments which should be passed to
p4 itself.  They will be available in $p4opts.  The command name will be
available in $p4command.  Arguments to the command will be available in
the positional parameters as usual.

Three custom commands are included in this commit.

"p4 diffchange" does a diff of only files opened against the default
changelist or the changelist specified by the -c flag.

"p4 login" attempts to renew Kerberos credentials if SSO login is
enabled.

"p4 update" detects all new, delete or changed files, like "git -A" does.