Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Implemtn getdtablecount() to count open file descriptors for current process.

Use underlying sysctl implemented by mjg in r290473.

PR: 194985
Reviewed by: bapt, mjg
Differential Revision: https://reviews.freebsd.org/D4084