Disable some checks for linux ports.

• shebang check: skip scripts in LINUXBASE. Things like /bin/bash are valid in a Linux context. Also, these scripts usually only work with a Linux shell so it doesn't make sense to try to make them work from a FreeBSD shell.
• prefixvar check: allow LINUXBASE/var.
• proxydeps check: only test FreeBSD ELF files because ldd(1) doesn't work with other ELF files.

PR: 212419
Submitted by: tijl
Sponsored by: Absolight