devel/p4 devel/p4api: cache distfiles in LOCAL

Perforce unhelpfully rebuilds their distfiles every few hours, changing the
archived files' timestamps in the process. This obviously changes their
checksums. Work around the issue by caching local copies of the distfiles and
using those for the checksums.

Also, set a sane DIST_SUBDIR since the distfiles are ambiguously named.

Also, take over maintainership from gordon.

Reported by: gordon, antoine
Reviewed by: mat
Approved by: mat (ports)
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D14211