Fix main_ctx_init: remove premature dir creation, increase MAX_PATH, add config file fallback

[Copilot]

main_ctx_init eagerly creates a logs/ directory and sets the log file before callers can customize paths. Config file lookup is also hardcoded to a single location.

• MAX_PATH: 260 → 1024. The Windows-era default was too small for typical Unix paths.
• Remove premature side effects: Removed hv_mkdir(logdir) and hlog_set_file() from main_ctx_init. Callers can now customize g_main_ctx.logfile before the log directory is created.
• Config file fallback chain: Search for the config file across multiple standard locations instead of assuming run_dir/etc/:
  1. run_dir/etc/{program}.conf
  2. run_dir/{program}.conf
  3. /usr/local/etc/{program}.conf (Unix)
  4. /etc/{program}.conf (Unix)
  5. Falls back to run_dir/etc/{program}.conf if none found

Original prompt

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This section details on the original issue you should resolve

<issue_title>Some issues about the function main_ctx_init</issue_title>
<issue_description>1. The length of the macro MAX_PATH is only 260, isn't it really too small?
2. The logdir directly reads the running directory, then creates and uses the directory logs in this directory. Even if the directory was modified later, an empty directory has already been created here.
3. The configuration file directory should have a rollback: first try the current directory (not the current directory/etc), then try the directory /usr/local/etc if it does not exist, and finally try the directory /etc.

The main issue is that the directory will be automatically created before customizing the path.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Some issues about the function main_ctx_init #549

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