allow RUST_BACKTRACE=0 to act as if unset

/# This is a combination of 16 commits.
/# The first commit's message is:
allow RUST_BACKTRACE=disabled to act as if unset

When RUST_BACKTRACE is set to "disabled" then this acts as if the env.
var is unset.

/# This is the 2nd commit message:

case insensitive "DiSaBLeD" RUST_BACKTRACE value

previously it expected a lowercase "disabled" to treat the env. var as
unset

/# This is the 3rd commit message:

RUST_BACKTRACE=0 acts as if unset

previously RUST_BACKTRACE=disabled was doing the same thing

/# This is the 4th commit message:

RUST_BACKTRACE=0|n|no|off acts as if unset

previously only RUST_BACKTRACE=0 acted as if RUST_BACKTRACE was unset
Now added more options (case-insensitive): 'n','no' and 'off'
eg. RUST_BACKTRACE=oFF

/# This is the 5th commit message:

DRY on the value of 2

DRY=don't repeat yourself
Because having to remember to keep the two places of '2' in sync is not
ideal, even though this is a simple enough case.

/# This is the 6th commit message:

Revert "DRY on the value of 2"

This reverts commit 95a0479d5cf72a2b2d9d21ec0bed2823ed213fef.

Nevermind this DRY on 2, because we already have a RY on 1,
besides the code is less readable this way...

/# This is the 7th commit message:

attempt to document unsetting RUST_BACKTRACE

/# This is the 8th commit message:

curb allocations when checking for RUST_BACKTRACE

this means we don't check for case-insensitivity anymore

/# This is the 9th commit message:

as decided, RUST_BACKTRACE=0 turns off backtrace

/# This is the 10th commit message:

RUST_TEST_NOCAPTURE=0 acts as if unset

(that is, capture is on)

Any other value acts as if nocapture is enabled (that is, capture is off)

/# This is the 11th commit message:

update other RUST_TEST_NOCAPTURE occurrences

apparently only one place needs updating

/# This is the 12th commit message:

update RUST_BACKTRACE in man page

/# This is the 13th commit message:

handle an occurrence of RUST_BACKTRACE

/# This is the 14th commit message:

ensure consistency with new rules for backtrace

/# This is the 15th commit message:

a more concise comment for RUST_TEST_NOCAPTURE

/# This is the 16th commit message:

update RUST_TEST_NOCAPTURE in man page

CONTRIBUTING.md

@@ -71,7 +71,8 @@ which includes important information about what platform you're on, what
 version of Rust you're using, etc.
 
 Sometimes, a backtrace is helpful, and so including that is nice. To get
-a backtrace, set the `RUST_BACKTRACE` environment variable. The easiest way
+a backtrace, set the `RUST_BACKTRACE` environment variable to a value
+other than `0`. The easiest way
 to do this is to invoke `rustc` like this:
 
 ```bash