BUILD: server-state: avoid using not-so-portable isblank()

Once in a while we get rid of this one. isblank() is missing on old
C libraries and only matches two values, so let's just replace it.
It was brought with this commit in 2.4:

  0bf268e18 ("MINOR: server: Be more strict on the server-state line parsing")

It may be backported though it's really not important.
1 file changed