Fix make headerscheck
In the wake of commits dac048f71 and ecaf7c5df, `make headerscheck`
no longer generated all headers that are included by other headers,
causing headerscheck/cpluspluscheck to fail. To fix, backpatch enough
makefile rules from 721856ff2 to generate all required headers.
Minor fixes to pg_combinebackup and its documentation.
The --tablespace-mapping option was specified with required_argument
rather than no_argument, which is wrong. Since the actual argument
string passed to getopt_long() included "T:", the single-character
form of the option still worked, but
