man: Misc syntax fixes

• loader.efi.8: use proper way of printing a backslash.
• usr.bin/gzip/gzip.1: contained a non-breaking space (in utf-8, 0xC2A0).
• lib/libpmc/pmc.*.3: remove two duplicate .Xr lines

Signed-off-by: Graham Percival <gperciva@tarsnap.com>
Reviewed by: mhorne, Alexander Ziaee <concussious.bugzilla@runbox.com>
MFC after: 3 days
Sponsored by: Tarsnap Backup Inc.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1523