ldso: add __L4_ThreadSwitch to ldso symbol.map

This wasn't necessary before because we built an l4 library for
Pistachio and linked it against each application. With the new linker,
we compile the required files from within Genode and create a syscall
library that is only linked to ldso. If a program uses system calls
directly, for example, DDE kit's spinlock implementation, the required
symbols must be made globally accessible.

Fixes #1306
Fix compiler warning about uninitialized variable

Fixes #1051
libc: support F_SETFD && FD_CLOEXEC ioctl

Issue #1308
vbox: support routing serial output to Genode

Fixes #1308