Fix several issues with kgdb:

• The kld parsing code in kgdb was using basename() and dirname() which ended up being broken by the recent changes to make dirname() modify its input buffer. Modify fbsd-kld.c to use the lbasename() and ldirname() wrappers in gdb.
• Add the kgdb.1 manpage from base and install it as 'kgdbNNN.1' which is helpful since stock HEAD no longer ships a kgdb manpage.
• Link libkvm into mips binaries so that the vmcore target links.
• Bump PORTREVISION.

PR: 219028
Reviewed by: luca.pizzamiglio@gmail.com (maintainer)
Approved by: rene