doc: improve bpchar and character type length details

Implement find_my_exec()'s path normalization using realpath(3).

Make PG_TEST_NOCLEAN work for temporary directories in TAP tests

doc: document the TAP test environment variables

doc: update metacpan.org links to avoid redirects

Use min/max bounds defined by Zstd for compression level

Fix incorrect value for "strategy" with deflateParams() in walmethods.c

Add auxiliary lists to GUC data structures for better performance.

Replace the sorted array of GUC variables with a hash table.

Store GUC data in a memory context, instead of using malloc().

Make some minor improvements in memory-context infrastructure.