Replace some strtok() with strsep()

Replace some strtok() with strsep()

strtok() considers adjacent delimiters to be one delimiter, which is
arguably the wrong behavior in some cases.  Replace with strsep(),
which has the right behavior: Adjacent delimiters create an empty
token.

Affected by this are parsing of:

- Stored SCRAM secrets
  ("SCRAM-SHA-256$<iterations>:<salt>$<storedkey>:<serverkey>")

- ICU collation attributes
  ("und@colStrength=primary;colCaseLevel=yes") for ICU older than
  version 54

- PG_COLORS environment variable
  ("error=01;31:warning=01;35:note=01;36:locus=01")

- pg_regress command-line options with comma-separated list arguments
  (--dbname, --create-role) (currently only used pg_regress_ecpg)

Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: David Steele <david@pgmasters.net>
Discussion: https://www.postgresql.org/message-id/flat/79692bf9-17d3-41e6-b9c9-fc8c3944222a@eisentraut.org