make my_vsnprintf to use gcc-compatible format extensions

my_vsnprintf() is used internally in the server as a portable printf replacement. And it's also exported to plugins as a service.

It supports a subset of printf formats and three extensions:

• %`s means that a string should be quoted like an `identifier`
• %b means that it's a binary string, not zero-terminated; printing won't stop at \0, so one should always specify the field width (like %.100b)
• %M is used in error messages and prints the integer (errno) and the corresponding strerror() for it
• %T takes string and print it like %s but if the string should be truncated puts "..." at the end

gcc knows printf formats and check whether actual arguments match the format string and issue a warning if they don't. Unfortunately there seems to be no easy way to teach gcc our extensions, so for now we have to disable printf format checks.

An better approach would be to use gcc compatible format extensions, like Linux kernel does. We should migrate to a different syntax for our extensions

• %sI to mean "print as an identifier"
• %sB to mean "print a binary string"
• %uE to mean "print an errno"
• %sT to put a "..." as truncation indicator

Perhaps it'll also need %sS and %uU as synonyms for %s and %u, as a form of escaping. In case someone needs to print a string, followed by I — using %sI will not do it anymore, so %sSI can be used instead.

Old formats can still be supported or they can be removed. The major version of the service should be increased either way to signal an incompatible change (different semantics for, e.g., %sI).

All error messages and all usages of my_vsnprintf should be changed to use the new syntax. One way to do it is to disable old syntax conditonally, only in debug builds. All gcc printf format checks should be enabled.