[runtime] Don't define ucontext_t for android if PLATFORM is not android.
Implement the ReadEntityBodyMode properties
Implement Request.Abort()
Merge pull request #1353 from martinjt/ReadEntityBodyMode

Implement the ReadEntityBodyMode properties
Merge pull request #1354 from martinjt/RequestAbort

Implement Request.Abort()
Removed erroneously added property to HttpRequestBase in pull #1353
[sgen] Fix allowance calculation.

The formula for calculating allowance was not only overly complicated
but completely broken, which resulted in the allowance always being the
minimum (4 times the nursery size).  That resulted in too frequent garbage
collections when the heap grew larger.

Now we do something very similar to what Boehm does: Allow the heap
to grow by a third before the next major collection happens.
[runtime] Clean up the signal handling code by moving the signal handler signature definitions to mono-signal-handler.h and using them everywhere.
Fix a typo.
[jit] Remove some unused definitions.
Fix the build after the signals changes.
[sgen] Cleanup some allocation code
[sgen] Utility function to compute number of unfaulted pages.
[sgen] Add internal allocator default sizes usage statistic
[sgen] Add assertion on power of 2 for internal allocator section size
[sgen] Adjust internal allocator block size to slot size.

The internal allocator used size segregated blocks which were always
16k large.  Since we have quite a few different size classes this led
to a lot of allocation even for very small programs.

We now use block sizes that are determined by the slot size and are as
small as possible.  They're power-of-two multiples of the page size,
so we end up with blocks of 4k, 8k and 16k.
Fix a warning.
[jit] Add some micro-optimizations to speed up the AOT compiler.
Merge pull request #1356 from martinjt/ReadEntityV2

Removed erroneously added property to HttpRequestBase in pull #1353
[sgen] Fix assertion error on 32bits