[zircon][syscalls] Make zx_time_t and zx_duration_t signed types

This change is part of a hard transition and breaks the layers above.

zx_time_t and zx_duration_t are now int64_t. ZX_TIME_INFINITE is now
INT64_MAX.

A negative time value passed to a syscall is treated as any other time
value that's "in the past".

Test: CQ; new unit tests; built and ran topaz system on Eve

ZX-2100 #comment

Change-Id: I2a020cb2cf3dd7212608e24c1ee90cec58dd51ef

[time] Introduce zx_clock_t

Change-Id: Ice44dca050b2ae64b78c80e9aab042dd450b9a7a

[kernel][syscalls] Remove redundant filename prefix

All of these files are already in a directory named syscalls.

Change-Id: If4945b107c2ae9787e63dfbfe3750d7dfdba739b