syscall_emul: refactor FDEntry and children classes
syscall_emul: refactor FDEntry and children classes

Strips off superfluous inline specifiers in header file.

Changes return types and parameters that deal with strings
to include the const& specifiers.

Change-Id: I570b1801abb059b91216f0223458d00963b9ca1c
Reviewed-on: https://gem5-review.googlesource.com/12111
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu