Problem: Still old variant of strdup has been used (#691)

pgcopydb recently switched to Boehm-Demers-Weiser Garbage Collector[1] and replaced
malloc family with GC_malloc and friends. However, strdup is not replaced
to GC_strdup.

Solution: Use macros to replace strdup/strndup with it's GC variant.

[1] https://github.com/dimitri/pgcopydb/pull/647



Signed-off-by: Arunprasad Rajkumar <ar.arunprasad@gmail.com>