Extend sendFileWithContent() to handle custom content length in basebackup.c

Extend sendFileWithContent() to handle custom content length in basebackup.c

sendFileWithContent() previously got the content length by using
strlen(), assuming that the content given is always a string.  Some
patches are under discussion to pass binary contents to a base backup
stream, where an arbitrary length needs to be given by the caller
instead.

The patch extends sendFileWithContent() to be able to handle this case,
where len < 0 can be used to indicate an arbitrary length rather than
rely on strlen() for the content length.

A comment in sendFileWithContent() mentioned the backup_label file.
However, this routine is used by more file types, like the tablespace
map, so adjust it in passing.

Author: David Steele
Discussion: https://postgr.es/m/2daf8adc-8db7-4204-a7f2-a7e94e2bfa4b@pgmasters.net