Refactor pipe_read_line to return the full line

Refactor pipe_read_line to return the full line

Commit 5b2f4afffe6 refactored find_other_exec() and in the process
created pipe_read_line() into a static routine for reading a single
line of output, aimed at reading version numbers.  Commit a7e8ece41
later exposed it externally in order to read a postgresql.conf GUC
using "postgres -C ..".  Further, f06b1c598 also made use of it for
reading a version string much like find_other_exec().  The internal
variable remained "pgver", even when used for other purposes.

Since the function requires passing a buffer and its size, and at
most size - 1 bytes will be read via fgets(), there is a truncation
risk when using this for reading GUCs (like how pg_rewind does,
though the risk in this case is marginal).

To keep this as generic functionality for reading a line from a pipe,
this refactors pipe_read_line() into returning an allocated buffer
containing all of the line to remove the risk of silent truncation.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/DEDF73CE-D528-49A3-9089-B3592FD671A9@yesql.se