fix: do not exit() in create_pgdir()

This could leave the database in a locked state (pg_start_backup()).
And since all calls to create_pgdir() handle the return value correctly
we simply replace the exit() by a return false
fix: do not exit after pg_start_backup() w/o pg_stop_backup()
fix: remove own node earlier if force is set

We have to remove our own node before we check for a new master if force
is set; else master register would fail on the second time since there
already is a master (ourselves), even if we specify -F
fix: correctly check the return value of PQexec()

not only check if return value is not NULL but also check that the
returned result is a PGRES_COMMAND_OK (e.g. the INSERT was successful)
fix: create data directory after sanity check
fix: do not compare pointers but the strings
white space cleanup