std: Ignore a flaky test on freebsd

This test runs successfully manually, but the bots are having trouble getting
this test to pass. Ignore it on freebsd for now.
core: Allow kinds derivings
libstd: Check TMPDIR for android as well and if not set use '/data/local/tmp' instead of '/data/tmp'.
auto merge of #14053 : FlaPer87/rust/master, r=huonw

This patch allows for using derivings for `kinds` in libcore

r? @alexcrichton
auto merge of #14046 : alexcrichton/rust/ignore-a-test-on-freebsd, r=kballard

This test runs successfully manually, but the bots are having trouble getting
this test to pass. Ignore it on freebsd for now.
auto merge of #14054 : luqmana/rust/at, r=alexcrichton

`/data/local/tmp` seems to be more common.
Add ToCStr impl for &Path and ~str

This is a stopgap until DST (#12938) lands.

Until DST lands, we cannot decompose &str into & and str, so we cannot
usefully take ToCStr arguments by reference (without forcing an
additional & around &str). So we are instead temporarily adding an
instance for &Path and ~str, so that we can take ToCStr as owned. When
DST lands, the &Path instance should be removed, the string instances
should be revisted, and arguments bound by ToCStr should be passed by
reference.

FIXMEs have been added accordingly.
Process::new etc should support non-utf8 commands/args

The existing APIs for spawning processes took strings for the command
and arguments, but the underlying system may not impose utf8 encoding,
so this is overly limiting.

The assumption we actually want to make is just that the command and
arguments are viewable as [u8] slices with no interior NULLs, i.e., as
CStrings. The ToCStr trait is a handy bound for types that meet this
requirement (such as &str and Path).

However, since the commands and arguments are often a mixture of
strings and paths, it would be inconvenient to take a slice with a
single T: ToCStr bound. So this patch revamps the process creation API
to instead use a builder-style interface, called `Command`, allowing
arguments to be added one at a time with differing ToCStr
implementations for each.

The initial cut of the builder API has some drawbacks that can be
addressed once issue #13851 (libstd as a facade) is closed. These are
detailed as FIXMEs.

Closes #11650.

[breaking-change]
Change dynamic_library::open_external to take ToCStr

`std::unstable::dynamic_library::open_external` currently takes a
`Path`, but because `Paths` produce normalized strings, this can
change the semantics of lookups in a given environment. This patch
generalizes the function to take a `ToCStr`-bounded type, which
includes both `Path`s and `str`s.

Closes #11650.