Add sbuf streaming mode to pseudofs(9), use in linprocfs(5)

Add a pseudofs node flag 'PFS_AUTODRAIN', which automatically emits sbuf
contents to the caller when the sbuf buffer fills. This is only
permissible if the corresponding PFS node fill function can sleep
whenever it appends to the sbuf.

linprocfs' /proc/self/maps node happens to meet this requirement.
Streaming out the file as it is composed avoids truncating the output
and also avoids preallocating a very large buffer.

Reviewed by: markj; earlier version: emaste, kib, trasz
Differential Revision: https://reviews.freebsd.org/D27047