Seven of the eight other relkind codes are lower-case, so it wasn't
consistent for this one to be upper-case.  Fix it while we still can.

Historical notes: the reason for the lone exception, i.e. sequences being
'S', is that 's' was once used for "special" relations.  Also, at one time
the partitioned-tables patch used both 'P' and 'p', but that got changed,
leaving only a surprising choice behind.

This also fixes a couple little bits of technical debt, such as
type_sanity.sql not knowing that 'm' is a legal value for relkind.

Discussion: https://postgr.es/m/27899.1488909319@sss.pgh.pa.us