Fix so MASTER_ID value written into mkdirs.N.cfg file is correct.
This fixes an issue where p4_N.vars files are generated with an
incorrect P4MASTER_ID value.
With this change, passsing the ServerID from install_sdp.sh to
called mkdirs.sh will no longer require the '-s <ServerID>' nor
the '-t <ServerType>' command line options.
Also change text in generated config file to indicate that it is OK
to set the ServerID of a commit/master server if you have a desired
ServerID that is different from the SDP-default (currently master.N).
Adjusted terminology to prefer 'p4d_commit' over 'p4d_master', but
preserve p4d_master as a synonym for backward compatibility with older
config giles.