Fix more strcmp() calls using boolean-like comparisons for result checks

Fix more strcmp() calls using boolean-like comparisons for result checks

Such calls can confuse the reader as strcmp() uses an integer as result.
The places patched here have been spotted by Thomas Munro, David Rowley
and myself.

Author: Michael Paquier
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/20190411021946.GG2728@paquier.xyz