[ulib][musl] Use the correct clock types

Change-Id: Ibeb116be3ae15e7ae869ab0a96a4d1f5d77dd8de

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[time] Switch callers of syscalls with timeouts to use conversion function

This is in preparation for moving syscalls to absolute deadlines rather
than relative timeouts.

Change-Id: Ic85abbf76f34fbce7899c2505be840e3d3dcf7ff

[ulib][magenta][runtime][musl] Give system calls _mx_* names, use them in musl

The mx_* name space is not kosher for the standard C library to use
for its own purpose. A conforming standard C program can define its
own functions called mx_* and this must not break the C library.

Give all the system call entry points in the vDSO names starting
with _mx_, with the old mx_ names as aliases. Use only the _mx_
names in runtime and musl.

Change-Id: Id01b57b87c0bb91733e9492159401ff019943822

[syscalls][api] rename _magenta_*() to mx_*()

The _magenta_ prefix is pretty unwieldy.
This change simplifies that a bit.

This was done mechanically, and boots, runs, and passes tests
on qemu arm64, amd64, and NUC.

Change-Id: I8eb07db6ed6f4b8cac96344cc4ad65ce3f85f917

[musl] Remove SYS_nanosleep in favor of _magenta_nanosleep

Additionally implement the time-remaining semantics for nanosleep based
on the current semantics of _magenta_nanosleep.

This also introduces a single place for time conversions. Over time we
should migrate all time conversion to one well validated set of routines.

Change-Id: I0930d95dbb1b94c160f40eedea64d47eb6fd8e1b

[magenta] Initial commit