RFR: 8345012: os::build_agent_function_name potentially wastes a byte when allocating the buffer [v4]

[David Holmes]

> This is a minor issue noticed in the review of JEP 479. The calculations of the buffer length always do `+2` to allow for an optional underscore plus the terminator. A solution was suggested in that review (which I initially used) but I've instead opted for a solution where `name_len` always reflects the actual length of the lib name - otherwise we need to subtract the 1 again when doing the `strncat`. I also made the need for the underscore explicit. 
> 
> As the same code is used in os_posix.cpp both versions were made consistent. The only difference between them is the Windows version has to check for a drive specifier. I toyed with combining them into a shared version with a Windows-specific chunk, but opted for the simpler change. I can revisit that if people have strong opinions.
> 
> Testing
> - tiers 1-4
> 
> Thanks

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  Move to shared code and apply @shipilev suggestions

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/22404/files
  - new: https://git.openjdk.org/jdk/pull/22404/files/be254520..bcee0a57

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22404&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22404&range=02-03

  Stats: 166 lines in 3 files changed: 59 ins; 107 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22404.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22404/head:pull/22404

PR: https://git.openjdk.org/jdk/pull/22404