pgsql: Add new error code for "file name too long"

Add new error code for "file name too long"

This new error code, named file_name_too_long, maps internally to the
errno ENAMETOOLONG to produce a proper error code rather than an
internal code under errcode_for_file_access().  This error code can be
reached with some SQL command patterns, like a snapshot file name.

Reported-by: Alexander Lakhin
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/Zo4ROR9mgy8bowMo@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2d8ef5e24fd3d38ffc2379ca32eaeef8c9eee1a7

Modified Files
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src/backend/utils/errcodes.txt | 1 +
src/backend/utils/error/elog.c | 4 ++++
2 files changed, 5 insertions(+)