PMC: Add --maxfds to control size of FDCache

This commit adds a --maxfds argument to PMC that enables the user
to set the maximum number of file descriptors that will be cached
by PMC in I/O forwarding. This is to help SCEC accomplish coscheduling
on BlueWaters.
PMC: Update manpage to document --maxfds
PMC: Add logging messages for current open files