pgsql: Rename function to first_path_var_separator() to clarify it work

Rename function to first_path_var_separator() to clarify it works with
path variables, not directory paths.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=426227850be145b52cb610455000fc005df5ce4e

Modified Files
--------------
src/backend/utils/fmgr/dfmgr.c | 2 +-
src/include/port.h | 2 +-
src/port/exec.c | 2 +-
src/port/path.c | 4 ++--
4 files changed, 5 insertions(+), 5 deletions(-)