Refactor code from custom bool_t to stdbool.h (where available)

[jimklimov]

Currently many places in codebase copy around a definition of a bool_t as enum or int; there are few uses of stdbool.h that should have handled the type.

I suppose we have legacy systems to consider that might not have this header yet, so maybe a nut_stdbool.h (similar in purpose and construct to timehead.h and nut_stdint.h) is in order, to streamline this basic type definition and use.

UPDATE: See also, some prior art found:

• Issue Consolidate bool_t definitions into a common header file #31
• Commit zykh@222d6e0 (and likely a sibling in libusb-compat-1.0 branch)
• Also noted in FTY branch differences from master, see Upstreaming improvements from 42ity/nut fork #1316
• Similar concern about common handling of boolean-ish strings in one implementation, see Consolidate bool string parsing into a common method #2811
• Rename bool union member wmnut#11 gives another interesting reminder from @a3a3el :

In C23, bool has been changed from a macro defined in <stdbool.h>, which expands to _Bool, to a keyword.
(Further note concerns wmNut code, but it is worth checking other repos including NUT for similar cases; perhaps extend configure.ac to detect the kind of bool definition a build environemnt has or does not have) The var union defined in wmgeneral.h contains a member named bool, which is, therefore, not compatible with C23. The next version of gcc defaults to C23, and so this union member will cause compilation to fail. Rename it to boolean.

• Replace if (cond) { x = 1; } else { x = 0; } with x = cond;