Keep file descriptors inherited by default

[belovdv]

From #64:

Then jobserver is initialized from pipe, it sets cloexec, but doesn't delete environment variable which points to file descriptors. By default (unless Client::configure is used), child process will be instructed (if it uses jobserver) to access file descriptors that was closed and maybe reopened to point to other files.

This seems like a violation of IO-safety.

Either cloexec shouldn't be set or the environment variables should also be removed and only be added back via configure.

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This pr is aimed to implement first option.

Concerns:

• current behavior is how gnu make works,
• it may be breaking change.

Reasons to choose first option:

• in a typical case inheritance is what we want - inheritance by default will reduce amount of places in code where jobserver code is called,
• fifo-style (named pipes) jobserver is inherited by default:
• • it's weird to have different behaviors depending on implementation details,
• • modern posix gnu make uses this fifo style by default, so it may be more common in the future,
• second option requires changing (more) global things (cloexec, environment variable).

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Concerns:

• with inheritance by default, how to work with multiply jobservers at once, if it would be necessary?
• with inheritance by default, Client::new in current pr's code changes global state - env var.

Notes:

• pr is a draft, in particular, documentation wasn't updated,
• windows implementation creates semaphore without SECURITY_ATTRIBUTES::bInheritHandle and does nothing in Client::configure. According to learn.microsoft.com, the handle cannot be inherited by child processes.