upstream: s/process_mux_master/mux_master_process/ in mux master

function names,

Gives better symmetry with the existing mux_client_*() names and makes
it more obvious when a message comes from the master vs client (they
are interleved in ControlMaster=auto mode).

no functional change beyond prefixing a could of log messages with
__func__ where they were previously lacking.

OpenBSD-Commit-ID: b01f7c3fdf92692e1713a822a89dc499333daf75
