pgsql: Add macros to check if a filename is a WAL segment or other such

Add macros to check if a filename is a WAL segment or other such file.

We had many instances of the strlen + strspn combination to check for that.
This makes the code a bit easier to read.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/179cdd098196338880bdbb39c39a788abdad4dd8

Modified Files
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src/backend/access/transam/xlog.c      |   11 +++--------
src/backend/replication/basebackup.c   |    7 ++-----
src/bin/pg_basebackup/pg_receivexlog.c |   16 ++--------------
src/bin/pg_resetxlog/pg_resetxlog.c    |    8 ++++++--
src/include/access/xlog_internal.h     |   18 ++++++++++++++++++
5 files changed, 31 insertions(+), 29 deletions(-)